The present invention relates generally to overvoltage surge arresters which have a plurality of internal component units stacked inside a housing between conductive terminal members and relates more particularly but not exclusively, to such arresters which have as some of those units zinc oxide compound varistors surrounded by a thermally conductive electrically insulating resilient carrier for improving the thermal conductivity between the varistors and the housing.
Varistors of the zinc oxide compound type are sensitive to heating. With increasing temperature, their leakage current at a given voltage increases. This increased leakage current further raises the temperature, with the consequence that at a critical temperature and voltage condition the varistors become subject to a thermal runaway condition and fail by passing ever-increasing current. It is known that the thermal runaway condition can be minimized by improving the transfer of heat generated by leakage current in the varistor and by surge currents which may occur at times by improving the thermal coupling between the varistors and the housing porcelain. This can be done by surrounding each of the varistors with a thermally conductive electrically insulating material collar having a perimeter portion matching the inside wall contour of the housing and by making thermal contact between the collar and the housing wall. The insulating material may be, for example, a room temperature vulcanizing silicone rubber which is filled with an aluminum oxide sand containing coarse and fine grit.
The thermal contact between the collar and the wall may be made by stress fitting the collar in the housing. However, when the collar is highly filled with particulates to improve its own thermal conductivity, there is a dramatic decrease in its resilience. Without the needed resilience, it becomes necessary to require a low tolerance for mechanical dimension variations in the collar as well as in the housing in order to have a reliable stress fit. Such a tolerance requirement would signicantly increase manufacturing costs.